[PATCH] mm/ptdump: always stabilise against page table freeing using init_mm

[Lorenzo Stoakes]

x86 and arm64 invokes ptdump_walk_pgd() with non-init_mm mm whilst still
walking kernel page table ranges.

For x86 this is done in ptdump_curknl_show() and ptdump_efi_show(), the
first passing current->mm, and the second passing efi_mm (we reach kernel
mappings that init_mm protects for current->mm due to x86 cloning shared
kernel page tables for arbitrary mm's).

arm64 does so via ptdump_debugfs_register(), configured by efi_ptdump_info
for efi ranges against efi_mm.

The init_mm mmap lock is used to stabilise page table freeing against
ptdump, so take a nested lock on init_mm to ensure that we are correctly
stabilised.

We take this after mmap write locking the non-init_mm mm. Nothing acquires
the init_mm lock first before locking an arbitrary mm, so no deadlock is
possible.

Other fixes have been sent which update the two cases which can cause races
with ptdump in init_mm ranges to acquire the init_mm mmap write lock - vmap
and x86 CPA huge page promotion.

For arm64, commit fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump")
already provides exclusion against init_mm for the vmap case, which this
patch also pairs with.

The first point at which ptdump can race kernel page table freeing is
commit b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page
table"), so we target this in the Fixes tag.

Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
---
 mm/ptdump.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/ptdump.c b/mm/ptdump.c
index 973020000096..6bef47b1a073 100644
--- a/mm/ptdump.c
+++ b/mm/ptdump.c
@@ -178,11 +178,18 @@ void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd)
 
 	get_online_mems();
 	mmap_write_lock(mm);
+	/* To stabilise page tables we must hold the init_mm lock too. */
+	if (mm != &init_mm)
+		mmap_write_lock_nested(&init_mm, SINGLE_DEPTH_NESTING);
+
 	while (range->start != range->end) {
 		walk_page_range_debug(mm, range->start, range->end,
 				      &ptdump_ops, pgd, st);
 		range++;
 	}
+
+	if (mm != &init_mm)
+		mmap_write_unlock(&init_mm);
 	mmap_write_unlock(mm);
 	put_online_mems();
 

---
base-commit: a635d6748234582ea287c5ffeae28b9b23f91c7e
change-id: 20260710-b4-fix-non-init_mm-ptdump-016b88e2a2a6

Cheers,
-- 
Lorenzo Stoakes <ljs@xxxxxxxxxx>